Since the discovery of the sulfonamide herbicides comprising the sulfonylureas and triazolopyrimidines, more than two dozen sulfonylurea and close to a half dozen triazolopyrimidine herbicides have been commercially developed for selective weed control in a wide variety of crops (The Pesticide Manual, Twelfth Edition, C. D. S. Tomlin, ed., British Crop Protection Council, Surrey, U.K, 2000). As the mode of action of these sulfonamide herbicides is inhibition of the enzyme acetolactate synthase (ALS) found in plants but not animals, sulfonamide herbicides provide a valued combination of excellent efficacy against weeds with low use rates and very low toxicity to animals.
Sulfonamide herbicides like other agricultural chemicals can be formulated as concentrates in a variety of different forms, including liquid compositions such as emulsifiable concentrates and solid compositions such as wettable powders and granules. Granular compositions can be conveniently transferred and measured like a liquid, but unlike liquids, very little residue adheres to the walls of the product container. Furthermore, organic solvents and vapors are avoided. Compared to wettable powders, granules are relatively dust-free. A particularly useful type of granules are those which are water-dispersible. Water-dispersible granules, sometimes described as “dry flowables”, readily disintegrate when added to water to form a solution or suspension, which can then be sprayed on the locus to be treated. It is also advantageous for granular compositions to have good attrition resistance, low tackiness, and uniform bulk density.
Water-dispersible granules can be manufactured by a variety of processes, including fluid-bed granulation, pan granulation, spray drying, intensive mixing, compaction, paste extrusion and heat extrusion (such as melt extrusion). The physical dimensions and porosity of water-dispersible granules depends upon the manufacturing process used. Fluid bed granulation, spray drying and intensive mixing give granules that very rapidly break up and disperse in water because of granule dimensional properties such as small size, irregular surface and porosity. On the other hand, paste extrusion and heat extrusion provide granules of relatively consistent diameter and shape. The consistent diameter of extruded granules makes them useful in uniform blends as described in U.S. Pat. No. 6,022,552.
Granule composition is an important factor for obtaining sufficiently rapid dispersion of extruded granules. The dispersed particles formed on dilution should be no larger than 50 microns in their largest dimension to avoid premature settling, which may result in uneven application of the pesticide. It is therefore necessary that all of the components of the formulated product rapidly and completely disperse or dissolve in the dilution water. (If all of the components completely dissolve, then they can be regarded as being dispersed at the molecular level.) Water dispersibility of granules is determined not only by the composition of the granules but also by the composition and other properties of the aqueous medium to which the granules are added. For example, low temperatures and high concentrations of solutes can greatly retard granule disintegration.
Extruded granules are often most conveniently and cost-effectively prepared through paste extrusion using water to plasticize a powder mixture, which is then dried after extrusion. Paste extrusion avoids need for including binders that soften at elevated temperatures, as is required for heat extrusion. However, the use in paste extrusion of water as a plasticizer precludes inclusion of water-activated gas-generating ingredients, which otherwise can be used for accelerating disintegration and dispersion of heat-extruded or compacted granules.
Besides achieving satisfactory granule disintegration and dispersion, spray equipment clean-out can also be important. As sulfonamide herbicides comprise a highly active class of herbicides, it is desirable to clean out spray equipment before the equipment is subsequently used to treat a crop sensitive to the sulfonamide herbicide used in the previous application. Clean-out may require a rinsing procedure that is time-consuming and results in wastewater requiring proper environmental disposal. Furthermore, clean-out can be affected if the spray equipment contains organic deposits remaining from previous crop protection chemical applications or from other chemicals tank-mixed with the sulfonamide herbicide composition.
PCT Patent Application Publication WO 93/16596 describes a method for reducing residual sulfonylurea herbicide contamination of spray equipment by requiring as the first step the formulation of the sulfonylurea active ingredient in the form of an agriculturally suitable water soluble salt. Although a variety of methods are known for preparation of salts of sulfonamide herbicides from the corresponding free acid forms, as processes to prepare sulfonamide herbicide active ingredient often provide the free acid form either directly or as part of isolation, conversion to a salt would require an additional process step. Preferable would be formulations with improved spray equipment clean-out properties whereby the free acid form of the sulfonamide herbicide is directly used in the formulation process.
Now discovered is a process for conveniently preparing paste-extruded granular sulfonamide herbicide formulations that not only have satisfactory water dispersibility but also improved spray equipment clean-out properties.